The present invention relates to a semiconductor device, and more specifically to a semiconductor device having sealed therein a semiconductor element which constitutes a memory circuit.
In general, the semiconductor elements are sealed by a sealing member such as a ceramic material, a glass or a plastic material (resin). Among these sealing members (hereinafter referring to as packages), the ceramic package made of a ceramic material contains uranium and thorium in amounts of several ppm. As mentioned, for example, in 16th Annual Proceedings of 1978 International Reliability Physics Symposium, Apr. 18-20, 1978, San Diego, U.S.A., however, these impurities emit alpha-particles causing the memory circuit to erroneously operate. Due to this fact, the reliability of the semiconductor elements is often greatly decreased.
The inventors of the present invention have analyzed the erroneous operation of the memory circuit caused by the alpha-particles as mentioned below.
Uranium and thorium release energy over a range of 4 to 9 MeV when they decay naturally. The energy distribution of alpha-particles emitted from the package, however, ranges from 0 to 9 MeV, since the alpha-particles generated in the package come into collision with the molecules before they reach the surface of the material.
The alpha-particles which have infiltrated into a silicon pellet excite the electrons and travel while gradually losing their energy. Therefore, the range of alpha-particles in the substance is in reverse proportion to the density of the substance and is proportional to the initial energy. In silicon, the alpha-particles excite the electrons to impart energy of 3.6 eV. Further, a typical 5 MeV alpha has a range of 25 .mu.m in silicon. The electrons excited in silicon give rise to the generation of holes, whereby pairs of electrons and holes are generated along the loci of alpha-particles. Here, if it is supposed that the energy of the alpha-particles is 5 MeV, the number of the excited electrons Ne is given by, EQU Ne.perspectiveto.5 MeV/3.6 eV=1.4.times.10.sup.6
This is equivalent to an electric amount of 0.22 picocoulombs. The electrons then diffuse along with the concentration gradient, and recombine to extinguish. However, when the electric charge is supplemented by the biasing and is not negligible as compared with the electric amount under a boundary condition in a pellet, the device operates erroneously. The erroneous operation takes place without deteriorating the physical properties of the element, and is hence often called soft error.
In order to prevent such erroneous operation, the package material should be purified to such a high concentration that it does not contain uranium or thorium. Usually, however, the package material is made of a composite material consisting of several raw materials, and may contain impurities introduced through manufacturing steps. Therefore, it is difficult to make the whole package using a material which does not contain uranium or thorium. Or even if such a material is obtained, the package will not be suited for practical application because of expensive manufacturing cost.